1. Field of Use
The present invention relates to timers and more particularly to real time clock apparatus utilized by a data processing system.
2. Prior Art
It is well known that many computer applications require precision time generation apparatus such as a real time clock or an accurate time delay for the supervision of assigned programming tasks and the like. In general, in those instances in which accurate periods are required, an external circuit is used to time the necessary periods.
The clock when started continues to time a period until it is completed and the time is up. This results in an interrupt to the central processing unit which references a software clock manager routine for updating a real time clock counter word stored in the processing unit's main memory.
The several types of real time clocks include a programmable real time clock wherein the actual required period is preprogrammed within the clock by hardware or software, a free running real time clock which runs continuously with the clock signalling the central processing unit at the end of each period and a time of day clock which provides the central processing unit with the actual time. One type of real time clock is disclosed in U.S. Pat. No. 4,099,255 which issued to Philip E. Stanley, et al, and is assigned to the same assignee as named herein.
For the most part, real time clocks of the type mentioned usually require operator intervention in setting up the time of day and increase system overhead in requiring the central processing unit to update counter values in main memory which can be readily converted to a time of day indication. When it is desired to provide periodic interrupts on a time of day basis, this normally results in a further increase in system overhead.
Accordingly, it is an object of the present invention to provide timer apparatus which will minimize the system overhead required to be expended to provide a time of day interrupt capability.
It is a further object of the present invention to provide timer apparatus which would provide variable system interrupts without further increases in system overhead.